The industry demand for increasing power density and lowering the height of power converters imposed the use of planar inductors and planar transformers. The continuous trend for lower voltages and higher current has set new challenges for power magnetic components such as transformers. In order to simplify and control the manufacturing process for power magnetic components, the windings are embedded or buried within multilayer PCB structures. In such applications the copper thickness is limited. This limitation will exclude applications wherein large currents are processed, which today is the growing trend. One solution to overcome this problem is to split the current and process each section of it before it is provided to the output. Because the power dissipated due to the DC impedance is proportional with the square of the current, splitting the current, for example in two sections will reduce by a factor of four the power dissipation due to the DC impedance. Another limitation comes from the semiconductor devices. The trend towards miniaturization has forced the design to use surface mounted, smaller packages for semiconductor devices. These devices will accommodate only a limited die size, i.e., a semiconductor layer or layers of limited size. As a result, such devices provide only a limited current capability.
In FIG. 2 appears a prior art approach of splitting the output current wherein several transformers are employed. The primaries 16, 20 and 24 of the transformers 10, 12 and 14 are in series and the currents in secondaries 18, 22 and 26 are processed in parallel. The secondary windings can be placed in parallel directly or paralleled after the rectifiers (not shown). This concept, also described in U.S. Pat. Nos. 5,990,776 and 6,046,918 of Jitaru, both incorporated herein by reference, offers several advantages. First it splits the output current, which is further processed (rectified) on parallel paths, before it unites at the output of the converter. By placing several transformers in series the voltage across each primary winding is decreased, and as a result the number of turns in the primary winding can be reduced. A reduced number of turns will decrease the leakage inductance, which is proportional with the square of the number of turns. The use of smaller transformer, and as a result, a smaller magnetic core, will allow a better cooling due to an increased core surface area to volume ratio, will decrease the eddy current losses in the magnetic core due to a thinner core, and will prevent the electromagnetic resonant losses associated with very large magnetic cores.
One major drawback of this concept is the fact that the magnetizing inductance is lower, leading to larger magnetizing current and as a result lower efficiency. This is due to the fact that the magnetizing inductance is proportional with the square of the number of turns, and the total magnetizing inductance for the magnetic structure from FIG. 2 is the summation of all the magnetizing inductances. If there are used “n” independent transformers each of them with a number of turns in primary “N”, the magnetizing inductance of the structure is Lm=nKN2.
There remains therefore a need for an improved magnetic component with a better core and winding relationship. In particular, there remains a need for a transformer structure that splits the secondary current for parallel processing, uses a small core wound with series-connected primary windings, and produces an increased magnetizing flux for higher efficiency.